In most cases, tickets cannot be bought in advance for the sites -- you have to get in line in the morning and get tickets for that day, usually on the site. At Giza, for example, tickets are available to enter the Great Pyramid only on the day you intend to climb inside, and then, only a certain number are sold each day. We had to jog from the main ticket kiosk (wher eyou get tickets to the site itself) across the plateau to the ticket office for the pyramid itself. Tickets are sold separately for the site, for each pyramid (if you want to go inside), the solar boar musuem, etc.
This is true for most sites, although tickets really aren't that expensive (from 5-10 pounds -- 1-2 dollars) for most things. An exepction is a ticket for any of the sound and light shows, which can run 30-40, depending on the location. Still, a pretty low price for access. In the Valley of the Kings/Queens/Nobles, tickets are sold for a series of three tombs each, with a separate ticket needed for the tomb of Tutankhamun and tomb of Nefertiri (which, btw, costs 100 pounds!)
I've listed the estimated entrance prices for most of the sites in the Cost section of the site, here.
Note that most places sell a separate ticket for your camera -- usually 5-10 pounds to bring in a camera and take NON FLASH pictures. Some will charge extra for a tripod, and a video camera can cost you upwards of 100 DOLLARS in some places. Check in advance if you can take in your video camera, or decide if you're willing to pay up. At the tombs (Amarna, Valley of the Kings, Beni Hassan, etc), the fee is ten pounds for EACH TOMB you want to take pictures in, althoughw e found that in most cases a single camera ticket was accepted at all the sites, if it was checked at all.